The present invention relates to an apparatus for converting the transmission mode of a microwave signal which is applied to a waveguide, amplifying the signal in the converted mode, and delivering the amplified signal to a subsequent circuit element via a microstrip line.
While the transmission mode of a microwave signal which is transmitted by a waveguide is generally a TE mode or a TM mode, that of a signal which is transmitted by a microstrip line is a TEM mode. To transmit a microwave signal from a waveguide to a microstrip line, therefore, the transmission mode of the signal is converted from the TE mode or the TM mode to the TEM mode while, at the same time, the signal is amplified to a suitable degree.
A prior art microwave amplifying apparatus constructed to output an amplified microwave signal from a waveguide via a microstrip line includes a waveguide having a pickup probe and shorted at one end thereof, and a mount joined with the waveguide and provided with an MIC base thereon. The waveguide is hollow and rectangular in cross-section. The MIC base is loaded with, for example, a first to a third microstrip lines, a first field effect tranistor (FET) adapted for amplification which is interconnected between the first and second microstrip lines, and a second FET for amplification interconnected between the second and third microstrip lines. Each of the microstrip lines is provided with a plurality of stubs for matching the impedance of the microstrip line to that of the FET.
The first microstrip line is built in to optimize the noise factor of the first FET which serves as a first-stage amplifier element of the amplifying apparatus. A problem brought about in this construction is that a dielectric loss is produced in the first microstrip line. As for a microwave signal lying in a 10 gigahertz frequency band, for example, a dielectric loss of approximately 0.2 decibel is observed for a length of 1 centimeter of the microstrip line. In this condition, the signal level applied to an input portion of the first-stage FET is lower than that which appears at an output terminal of the pickup probe, making the noise factor poorer correspondingly. Hence, the noise factor attainable with the prior art amplifying apparatus is not fully acceptable.